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Earth's Moon

The Moon at Last Quarter Phase
Credit: NSO

The Moon, also called Luna, is the Earth's only natural satellite, and the fifth largest satellite in the entire Solar System.

It is also the only object beyond Earth that humans have ever set foot on. This occurred back in 1969, when the astronaut Neil Armstrong climbed out of the Apollo 11 landing craft. Unlike the Earth, however, the Moon has a very thin atmosphere, which is far too thin for people to breathe. Combine that with the wide variation in surface temperature between the night and day sides of the Moon (approximately -170°C to 120°C) and you can see why visiting astronauts wore such cumbersome space suits.

The Moon's diameter is 3,474 km, which is a little over a quarter of the diameter of the Earth. The average distance from the Earth to the Moon is 384,403 km, or roughly 30 times the diameter of the Earth. Due to its smaller size and mass, the pull of gravity on the lunar surface is about 17% of that at the Earth's surface.

Because of the effect of tidal forces over many millions of years, which have caused the orbit and rotation of the Moon to alter, the Moon always has the same face towards us. This means that the only way to see the far side of the Moon is in a spacecraft.

If you look carefully at the Moon's surface, you will see that it's covered with craters and mountain ranges. The craters were caused by the impact of countless meteors over many millions of years. There are also extensive regions that appear dark and smooth, and which have far fewer craters. These are called Maria (the Latin word for Seas) because early astronomers thought that they were old oceans. We now know that they are regions of solid lava, left over from when the Moon was much younger and hotter inside.

The Moon makes a complete orbit around the Earth every 27.3 days. Because the Sun shines on the Moon from different angles during this time, it appears that it is changing shape. We call these changes the Phases of the Moon.

The Earth-Moon-Sun diagram on the left gives a clearer picture of the different lunar phases we can see.

However, since the Earth is also moving around the Sun during the Moon's orbit, it actually takes an extra 2.2 days to see the full cycle of lunar phases. This is because the Moon has to rotate a little bit more to be pointing at the Sun again. In other words, it takes 29.5 days to go from one full moon to the other.